In the context of wind turbines for power production, the number of revolutions of the blades is adjusted in accordance with the speed of the wind to the effect that as much wind power as possible is converted into electric power. To use to advantage as much as possible of the wind power, the blade tip speed describing the speed of the blade tip is to be adapted to the speed of the wind. This is accomplished by changing the rate of revolution of the blades in pace with the speed of the wind changing. When the wind power is exploited optimally, the speed of the blade tip is proportional to the speed of the wind, meaning that the blade tip speed is increased when the speed of the wind increases and is correspondingly lowered when the speed of the wind slows down.
Today, the blades of a wind turbine are constructed as a rigid rotor, which means that the individual blades in the plant all have the same rate of revolution. This means that the angle between two blades in a wind turbine with a total of three blades is 120°. The speed of the blades can be adjusted by stalling or pitching the blades to the effect that the angle of attack of the wind on the blade is optimised relative to the speed of the wind. When the blade is actively stall-adjusted, the angle of attack is changed to the effect that turbulent air flows are generated across the blade, and hereby the lift of the blade is reduced. Therefore, the blade will be braked when it stalls, but it can be adjusted back to optimise the angle of attack, and the blade will again achieve maximal lift. When the blade is pitch-adjusted, the angle of attack of the wind is changed in the opposite direction compared to a scenario in which active stall-adjustment is performed, and thereby the blade loses its lift, but turbulent flows are not formed across the blade. The rate of revolution of the rotor is adjusted in pace with the wind speed changing, and such adjustment typically takes place on the basis of a measurement of the speed of the wind by means of eg an anemometer. The speed of the wind is typically measured on the nacelle, ie at approximately the same height as the hub of the wind turbine, to the effect that the rate of rotation of the blades is determined on the basis of the wind speed at the hub.
However, the speed of the wind varies with the height above ground and will typically be increasing with increasing height above ground. Therefore each individual blade will meet different wind speeds during a rotation cycle, meaning that the blade is influenced by a relatively high wind speed at the top of the rotation cycle and a lower wind speed at the bottom of the rotation cycle. The rate of rotation of the blades being adjusted on the basis of wind speed at the hub, the blade will have a propensity to go too fast when it is situated in the upper half of its rotation cycle, and too slowly when it is situated in the bottom half of the rotation cycle, which causes forces in the hub. Today, in order to reduce such forces, the blades are individually pitch-adjusted (U.S. Pat. No. 6,604,907; WO2005090781). This is typically accomplished by pitching the blades out of the wind when they are located in the upper half of the rotation cycle, and thereby the lift on the blades is reduced, whereby the blade loses some of its momentum. It is a drawback of this system that the power of the wind cannot be used optimally when the blades are pitched out of the wind due to the speed of the blade tip no longer being optimal relative to the speed of the wind.
HAU E.: “windkraftanlagen” 1996, SPRINGER VERLAG, Berlin, DE, page 172-176 discloses that the blades of a two bladed wind turbine can have different degrees of freedom in order to reduce the loads on the wind turbine due to asymmetric outflow conditions. The degrees of freedom could for instance allow the blades to pendulant in and out of the rotor plane, to be pitched or to be angularly displaced in the rotor plane.
WO 2005/068833 A2 discloses a wind turbine where the pitch of the blades can be varied according to the wind velocity by using a combination of the wind speed and the centrifugal force. The blades are hinged such that they can be rotated/lifted in/out of the rotor plane. The pitch of the blades can hereby be varying according to the wind velocity and the centrifugal force.
FR 1025422 discloses a rotor for e.g. a helicopter or wind turbine. The blade comprises an outer blade part and an inner blade part fastened to the hub. The outer blade part is introduced into the inner blade part in a telescopic way and can be turned around an axis, which is parallel to the main shaft and displaced from the main shaft. The consequence is that the angular displacement in the rotor plane of the outer blade part can be varied.
None of the wind turbines and rotors disclosed in the above-mentioned documents provide sufficient reduction of the loads on the wind turbine, and it is further impossible to optimise the blade tip speed relatively to the speed of the wind.